Analog interfaces are used in a variety of digital circuit applications. A typical analog interface is shown in FIG. 1, and includes an anti-aliasing filter and an analog-to-digital (A/D) converter. An analog input voltage is fed to the anti-aliasing filter. The filtered voltage is then fed to the A/D converter and the A/D converter generates digital output signals. The anti-aliasing filter suppresses high-frequency components to avoid aliasing when the analog signal is sampled by the A/D converter.
This conventional "voltage mode" method usually requires linear capacitors; however, modern CMOS baseline fabrication processes (e.g., EPIC 3 and CS11S) do not include the double poly options necessary to create linear capacitors. Thus, the requirement of linear capacitors increases the number of process steps and therefore increases the cost.
In mixed-voltage applications, such as linecard circuits, it is cost-effective to integrate high-voltage and low-voltage circuits on the same chip. Using traditional voltage mode interface circuits, between high-voltage and low-voltage circuits, the high signal swing in the high-voltage circuits must be limited to prevent saturation of or damage to the low-voltage circuits. However, limiting the signal swing reduces the dynamic range of the high voltage circuits.
To overcome the above disadvantages, an alternative "current mode" interface, shown in FIG. 2, can be used. An analog input voltage is first fed to the voltage-to-current (V/I) converter. The output current is passed to the filter and the filtered current is supplied to the A/D converter. Since the interface processes currents instead of voltages, linear capacitors are not necessary, and a pure digital CMOS baseline process can be used without additional processing steps or cost. Also the input voltage to the V/I converter can be arbitrarily larger than the supply voltage of the V/I converter since the V/I converter can be designed to sense only a current swing. This arrangement allows high-voltage and low-voltage circuits to be integrated on the same chip at a low cost.
It would be desirable for an analog digital interface to include both a voltage-to-current conversion capability and a filtering capability, and which can be easily fabricated using a digital CMOS baseline fabrication process such as CS11S.